Input transient protection for electronic devices

ABSTRACT

A circuit board assembly which includes an electrically insulating layer, a conductive printed wiring layer formed on the surface of the electrically insulating layer and includes a plurality of conductive paths, a conductive trace on the electrically insulating layer and apparatus for dissipating a transient in addition to a surface mount resistor fixed in relation to the trace. In some forms of the invention the surface mount resistor has opposed generally planar lips. The trace may also be generally planar. In some cases the lower lips and the trace are generally parallel. The generally planar lips of the surface mount resistor may be closer to the trace than the thickness of the surface mount resistor. A single geometric plane may extend through substantially all of the lips and all of the trace. In some cases the lower surface of the lips and the lower surface of the trace are substantially coplanar. In some cases the upper surface of the lower lip and the upper surface of the trace are substantially coplanar. In other cases the lower surface of the lower lip and the lower surface of the trace are substantially coplanar and in addition the upper surface of the lip and the upper surface of the trace are substantially coplanar.

BACKGROUND OF THE INVENTION

The invention relates to electronic circuit boards and particularlypassive means for protecting against voltage transient in electroniccomponents. Alarm systems and many other electronic devices are attachedthrough wires to external devices. These wires are subject to voltagetransients that can lead to failure of individual components on thecircuit board. The problem is particularly acute with respect toresistors at an input for a given electronic circuit. Wires that areconnected to the input for the circuit, referred to as “external wires”have voltage transients in them. The usual failure mode of suchresistors is a changing value of the resistor as the transient arcs fromthe body of the resistor, burning off the film deposited on theresistor.

Although the present invention will be described in terms of resistorson the input of a circuit board, those skilled in the art will recognizethat the present invention may be utilized with other resistors as wellas other surface mounted passive components such as inductors andcapacitors. This is particularly true of such surface mounted componentshaving end caps.

The invention has particular application to intrusion as well asfire/smoke detection alarm systems. Examples of intrusion detectionsystems include those shown in the following United States patentshaving the same assignee as the present application:

-   U.S. Pat. No. 6,593,850 Wireless intrusion detector with test mode    issued on Jul. 15, 2003 to Addy.-   U.S. Pat. No. 6,188,318 Dual-technology intrusion detector with pet    immunity issued on Feb. 13, 2001 to Katz, et al.-   U.S. Pat. No. 4,939,359 Intrusion Detection System with Zone    Location issued on Jul. 3, 1990 to Freeman.-   U.S. Pat. No. 4,614,938 Dual channel pyroelectric intrusion detector    issued on Sep. 30, 1986 to Weitman.-   U.S. Pat. No. 4,360,905 Intrusion Alarm System for Use with    Two-Wire-Cable issued on Nov. 23, 1982 to Hackett.-   RE No. 30,288 Intrusion Detection System issued on May 27, 1980 to    Hackett.-   U.S. Pat. No. 4,035,798 Ultrasonic Intrusion Detection System issued    on Jul. 12, 1977 to Hackett.

Examples of such a fire/smoke detection system 14 include the apparatusshown in the following United States patents having the same assignee asthe present application:

-   U.S. Pat. No. 6,150,935 Fire Alarm System With Discrimination    Between Smoke And Non-Smoke Phenomena issued on Nov. 21, 2000 to    Anderson;-   U.S. Pat. No. 6,084,522 Temperature Sensing Wireless Smoke Detector    issued on Jul. 4, 2000 to Addy-   U.S. Pat. No. 5,781,291 Smoke Detectors Utilizing A Hydrophilic    Substance issued on Jul. 14, 1998 to So, et al.-   U.S. Pat. No. 5,764,142 Fire Alarm System With Smoke Particle    Discrimination issued on Jun. 9, 1998 to Anderson, et al.-   U.S. Pat. No. 5,726,633 Apparatus and Method for Discrimination of    Fire Types issued on Mar. 10, 1998 to Wiemeyer;-   U.S. Pat. No. 5,659,292 Apparatus Including a Fire Sensor and a    Non-Fire Sensor issued on Aug. 19, 1997 to Tice.-   U.S. Pat. No. 5,633,501 Combination Photoelectric and Ionization    Smoke Detector issued on May 27, 1997 to Amieshi, et al.-   U.S. Pat. No. 5,557,262 Fire alarm System with Different Types of    Sensors and Dynamic System Parameters issued on Sep. 17, 1996 to    Tice.-   U.S. Pat. No. 5,117,219 Smoke and Fire Detection System    Communication issued on May 26, 1992 to Tice, et al.-   U.S. Pat. No. 4,916,432 Smoke and Fire Detection System    Communication issued on May 26, 1992 to Tice, et al.-   U.S. Pat. No. 4,374,329 Smoke Detector with Test Apparatus issued on    Feb. 15, 1983 to Schoenfelder, et al.-   U.S. Pat. No. 4,316,184 Combination Combustion-Product Detector    issued on Feb. 16, 1982 to Nagel.-   U.S. Pat. No. 4,225,860 Sensitivity Controlled Dual Input Fire    Detector issued on Sep. 30, 1980 to Conforti.-   U.S. Pat. No. 4,097,851 Sensitivity Compensated Fire Detector issued    on Jun. 27, 1978 to Klein.-   U.S. Pat. No. 4,091,363 Self-contained Fire Detector with    Interconnection Circuitry issued on May 23, 1978 to Siegel, et al.-   U.S. Pat. No. 4,020,479 Fire Detector issued on Apr. 26, 1977 to    Conforti, et al.

The disclosures of the above referred to patents are incorporated byreference. Various embodiments of the present invention may utilizedifferent fire and smoke detecting apparatus as well as differentintrusion detection and storage apparatus.

A preferred technique for assembling circuit boards involves the use ofsurface mount components and wave soldering. The invention hasparticular application to surface mount components such as resistors.One example of a surface mount component is described in U.S. Pat. No.5,604,477 entitled Surface Mount Resistor and Method for Making Sameissued on Feb. 18, 1997. One specific technique for wave soldering isdescribed in U.S. Pat. No. 4,208,002 entitled Wave Soldering Systemissued on Jun. 17, 1980. The disclosures of these patents areincorporated by reference.

Typically, the components that will tolerate the voltage transients arerelatively high cost large axial mount components. Thus, the traditionalapproach to avoiding the problem increases the cost for the componentand increases the cost of assembly by complicating the assemblyprocedure. Alternatively, relatively expensive transient absorbers suchas transorbs or metal oxide varistors (MOV) may be used. Thus, thetraditional approaches to solving the problem increase the cost of theproduct.

SUMMARY OF THE INVENTION

It is an object of invention to minimize the cost of assembling, thecost of the components and simplify the manufacturing process forelectronic circuit boards.

It also an object of the present invention to provide apparatus thatwill allow the use of small low cost surface mount components to replacelarge axial mount components.

Still another object of the invention is to prevent small inexpensivesurface mount components from being destroyed by transients.

It is another object of the invention to provide a passive device whichwill isolate the internal circuitry of these electronic devices fromtransients.

It has now been found that these and other objects of the invention maybe attained in a circuit board assembly which includes an electricallyinsulating layer, a conductive printed wiring layer formed on thesurface of the electrically insulating layer and including a pluralityof conductive paths, a conductive trace on the electrically insulatinglayer and apparatus for dissipating a transient in addition to a surfacemount resistor fixed in relation to the trace.

In some forms of the invention the surface mount resistor has opposedgenerally planar lips. The surface mount resistor may have generallyplanar lips and the trace may also be generally planar. In some casesthe lower lips and the trace are generally parallel. The generallyplanar lips of the surface mount resistor may be closer to the tracethan the thickness of the surface mount resistor. A single geometricplane may extend through substantially all of the lips and all of thetrace. In some cases the lower surface of the lips and the lower surfaceof the trace are substantially coplanar. In some cases the upper surfaceof the lower lip and the upper surface of the trace are substantiallycoplanar. In other cases the lower surface of the lower lip and thelower surface of the trace are substantially coplanar and in additionthe upper surface of the lip and the upper surface of the trace aresubstantially coplanar.

In some cases the surface mount resistor has a height of t and thespacing between the lip and the trace is less than t. In other cases thespacing between the lip and the trace is no more than one half t.

The invention the also includes a circuit protection system whichincludes a conductive trace and apparatus for dissipating a transient, asurface mount passive component having opposed end caps, each of the endcaps includes a lip, each lip is disposed in closely spaced relationshipto the conductive trace.

In some cases the lips have respective edges thereof which are mutuallyparallel. Each lip may be generally planar and the trace may also begenerally planar. In some cases the lips and the trace are generallyparallel. In other cases an edge of each lip is approximately 0.01 inchfrom the trace. Each generally planar lip of the surface mount passivecomponent may be closer to the trace than the thickness of the surfacemount passive component.

The invention also includes an alarm system that includes a circuitboard assembly which includes an electrically insulating layer, aconductive printed layer formed on the surface of the electricallyinsulating layer and including a plurality of conductive paths, aconductive trace on the electrically insulating layer and apparatus fordissipating a transient, a surface mount resistor fixed in closelyspaced relation to the trace.

In some cases the surface mount resistor has opposed generally planarlower lips. The surface mount resistor may have opposed generally planarlower lips and the trace may also be generally planar. In some cases thesurface mount resistor has opposed generally planar lower lips, thetrace is also generally planar and the lower lips and the trace aregenerally parallel.

The generally planar lower lips of the surface mount resistor may becloser to the trace than the thickness of the surface mount resistor.The surfaces of the lips and the trace may be parallel. The generallyplanar lips of the surface mount resistor may be spaced no more than0.01 inch from the trace.

The invention also includes an electrically insulating layer, aconductive printed wiring layer formed on the surface of theelectrically insulating layer and including a plurality of conductivepaths, a conductive trace on the electrically insulating layer andapparatus for dissipating a transient, a first surface mount resistorfixed in closely spaced relation to the trace and a second surface mountresistor fixed in closely spaced relation to the trace.

The surface mount resistors may have opposed generally planar lips. Eachof the surface mount resistors may have generally planar lips and thetrace is also generally planar. In some cases all of the lower lips andthe trace are generally parallel. The generally planar lips of thesurface mount resistors may be closer to the trace than the thickness ofthe surface mount resistors.

A single geometric plane may extend through substantially all of thelips and all of the trace. The lower surface of the lips and the lowersurface of the trace may be substantially coplanar. The upper surface ofthe lower lips and the upper surface of the trace may be substantiallycoplanar. Both the lower surface of the lower lip and the lower surfaceof the trace may be substantially coplanar and in addition the uppersurface of the lips and the upper surface of the trace may besubstantially coplanar.

In some cases the surface mount resistor has a height of t and thespacing between each lip and the trace is less than t. In other casesthe spacing between each lip and the trace is no more than one half t.In some embodiments of the invention the distance between the first andsecond resistors is greater than the height of each resistor. In someembodiments the distance between the first and second resistors is atleast three times the height of each resistor.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood by reference the accompanyingdrawing in which:

FIG. 1 illustrating schematically a section of three surface mountresistors at the input of an electronic circuit with first and secondprotective traces disposed to protect the respective surface mountresistors. This view is taken along a plane that is parallel to theupper face of the associated circuit boar and spaced therefrom justsufficiently to clear the pads (not shown) on the printed circuit boardthat are soldered to the end caps of the resistor.

FIG. 2 is a partially schematic, broken away perspective view of aconventional surface mount resistor illustrating a portion with greaterdetail.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawing, there is shown in FIG. 2 a broken awayperspective view of a conventional surface mount resistor 1. Suchresistors, often called chip resistors, are substantially formed asparallelepipeds, and have generally U-shaped end caps 2, 4. The end caps2, 4 are typically manufactured to industry-standard specifications. Thenoted dimensions t for thickness or height as well as other dimensionsd, W, and c are controlled during the resistor manufacturing process toinsure mounting of the chip resistor on a printed circuit board isfacilitated. The lower face of each end connector 2, 4, having thedimension d, registers with associated pads (not shown) on a printedcircuit board to which they resistor is ultimately mounted. Theconventional surface mount resistor includes a protective coating 5 a,ceramic substrate 5 b, resistive film 5 c, inner electrode 5 d, nickelplating 5 e, solder plating 5 f, nickel plating 5 f, termination (outer)5 g, termination (barrier) 5 h, and termination (inner) 5 i. Wavesoldering procedures secure the resistor to the printed circuit board.

FIG. 1 illustrates the relationship between three such resistors 14, 18,26 and protective traces 22, 24 in accordance with the presentinvention. More specifically, a first conductor 12 is part of anelectronic circuit, such as an electronic security system and is alsoconnected to external circuitry (not shown). It is this first conductor12 that experiences transients and which is also connected to a firstsurface mount resistor having respective end caps 14 a and 14 b. Asecond conductor 16 connects the first surface mount resistor 14 to asecond surface mount resistor having end caps 18 a and 18 b. A thirdconductor 20 connects the second surface mount resistor 18 to theinternal circuitry of a given system. In addition, a third surface mountresistor having end caps 26 a and 26 b connects to external circuitryvia conductor 28 and internal circuitry via conductor. Thesystem/internal circuitry may be the circuitry of an alarm systemlocated on the same circuit board.

The apparatus and method in accordance with present invention avoids theabove-described problems by providing a different path for the transientarc. More specifically, as shown in FIG. 1, a first metal trace 24 isplaced under the first resistor 14 and a third resistor 26. A secondmetal trace 22 is placed under the second resistor 18. Each metal traceis preferably manufactured of the same conductive material as that usedin the rest of the printed circuit board. Typically this will be copperalthough aluminum and other conductive materials may be used. Each metaltrace 22, 24 is connected to ground or may be at some potential. It isonly essential that the trace 22, 24 as well as connections to the trace22, 24 be dimensioned and configured to enable the respective trace todissipate transients without damage to any part of the circuit. In apreferred embodiment each trace 22, 24 is disposed approximately 0.01inches from the pad of the resistor. This dimension is indicated as “X”in FIG. 1 (4 instances). Accordingly, a transient will arc from the oneof the end caps 14 a, 14 b on the resistor 14 to the trace 24. It isimperative that the dimension X be greater than the dimension t (theheight of the end cap 14 a, 14 b and essentially the height of the chipresistor 14). To provide an adequate margin of safety is preferable thatthe dimension X be at least one half the dimension t.

Similarly a transient at the second resistor 18 will arc to the trace 22in the manner described above. In each case arcing from the lower facesof the resistors 14, 18 respectively to the traces 22, 24 will occurbecause the distance is shorter than the distance from the film on topof the resistor body to the ground trace. Ordinarily, the pad or lowerface of the respective resistors 22, 24 will be planar and therespective traces 22, 24 will also be planar. The precise spacingbetween the trace and the resistor will vary for different applications.Considerations to be considered in determining the spacing include thephysical characteristics of the resistor, the physical characteristicsof the trace, and the nature of the transients typically encountered.

It is preferable that the trace, such as the trace 24, be disposed insubstantially coplanar relationship with the lower lip of the end caps2, 4. (It will be the understood that the end caps 2, 4 have a generallyU-shaped contour. The lowermost part thereof (as viewed in FIG. 2) isthe lower lip that is preferably coplanar with the trace 24.) As usedherein, the term “coplanar” means a plane (that is inherently infinitelythin) will extend throughout the entire extent of the lower lips and thetrace 24. In a preferred embodiment the lower lips and the trace areboth planar and the top surface of the lower lips and the top surface ofthe trace are coplanar and in addition the bottom surface of the lowerlips and the lower surface of the trace are coplanar. Thus, the surfacearea of the trace 22 exposed to the edge of the lower lip is maximized.

The chip resistors 14, 18 a respectively protected by traces 24, 22. Thethird chip resistor 26 is protected by the trace 24 which is the sametrace that protects the first resistor 14. An additional considerationin the layout occurs in this case. More specifically, the dimension Y(the distance between adjacent end caps on the first resistor 14 and thesecond resistor 26) must be even greater than the dimension X. Morespecifically, it is important that the dimension Y be greater than X andis preferable that they dimension Y be no less than three times thedimension t. Because the transient at the first resistor 14 and thetransient at the third resistor 26 may instantaneously be respectivelypositive and negative the risk of damage (in the risk of an arc from oneend cap to another end cap) as the result of the transients is muchgreater than the case where only one transient is present or even whento transients are both positive or both negative.

In one embodiment of the invention each of the three chip resistors 22,24, 26 is a three quarter watt 2H/2010. The dimension t for thiscomponent is 0.024 inch. In a preferred embodiment the dimension X isapproximately 0.010 inch. In a preferred embodiment of the dimension Yis at least 0.080 inch. It will thus be seen that a transient will bediverted from the body of the component to elsewhere that will not causedamage. In the case of alarm system is possible to use surface mountcomponents for zone inputs on alarm systems without the addition ofexpensive transient absorbers.

As used herein the dimension t shown in FIG. 2 is referred to as theheight of the resistor. Those skilled in the art will recognize thatthis measurement is widely used in the industry. Accordingly, for easeof description of the present invention that simplification has beenadopted.

Although the present invention has been described in terms of resistorson the input of a circuit board, those skilled in the art will recognizethat the present invention may be utilized with other surface mountedpassive components such as inductors and capacitors. This isparticularly true for such passive components that are constructed withend caps in the manner of the surface mount resistor described above.Although the trace 22, 24 has been referred to as a single band thatextends close to opposed edges of respective end caps, it will beunderstood that alternatively the trace may be formed in two elongatedstrips.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments of thisinvention without departing from the underlying principles thereof.Accordingly, it will be appreciated that this invention is alsoapplicable to other systems. The scope of this invention should,therefore, be determined only by the following claims.

1. A circuit board assembly which comprises: an electrically insulatinglayer; a conductive printed wiring layer formed on the surface of saidelectrically insulating layer and including a plurality of conductivepaths; a conductive trace on said electrically insulating layer andmeans for dissipating a transient; a surface mount resistor fixed inrelation to said trace.
 2. The circuit board assembly as described inclaim 1 wherein said surface mount resistor has opposed generally planarlips.
 3. The circuit board assembly as described in claim 1 wherein saidsurface mount resistor has a generally planar lips and said trace isalso generally planar.
 4. The circuit board assembly as described inclaim 1 wherein said surface mount resistor has a generally planar lowerlip, said trace is also generally planar and said lower lip and saidtrace are generally parallel.
 5. The circuit board assembly as describedin claim 2 wherein said generally planar lips of said surface mountresistor are closer to said trace than the thickness of said surfacemount resistor.
 6. The circuit board assembly as described in claim 2wherein said lips and said trace are parallel.
 7. The circuit boardassembly as described in claim 4 wherein a single geometric planeextends through substantially all of said lips and all of said trace. 8.A circuit board assembly as described in claim 7 wherein the lowersurface of said lips and the lower surface of said trace aresubstantially coplanar.
 9. A circuit board assembly as described inclaim 7 wherein the upper surface of said lower lip and the uppersurface of said trace are substantially coplanar.
 10. A circuit boardassembly as described in claim 7 wherein the lower surface of said lowerlip and the lower surface of said trace are substantially coplanar andin addition the upper surface of said lip and the upper surface of saidtrace are substantially coplanar.
 11. A circuit board assembly asdescribed in claim 10 wherein said surface mount resistor has a heightof t and the spacing between said lip and said trace is less than t. 12.A circuit board assembly as described in claim 11 wherein the spacingbetween said lip and said trace is no more than one half t.
 13. Acircuit protection system which comprises: a conductive trace and meansfor dissipating a transient; a surface mount passive component havingopposed end caps, each of said end caps including a lip, each lip beingdisposed in spaced relationship to said conductive trace.
 14. Thecircuit protection system as described in claim 13 wherein said lipshave respective edges thereof which are mutually parallel.
 15. Thecircuit protection system as described in claim 13 wherein each lip isgenerally planar and said trace is also generally planar.
 16. Thecircuit protection system as described in claim 13 wherein each lip is agenerally planar lip, said trace is also generally planar and said lipsand said trace are generally parallel.
 17. The circuit protection systemas described in claim 16 wherein an edge of each lip is approximately0.01 inch from said trace.
 18. The circuit protection system asdescribed in claim 13 wherein respective surfaces of said lips and saidtrace are parallel.
 19. The circuit protection system as described inclaim 18 wherein each generally planar lip of said surface mount passivecomponent is closer to said trace than the thickness of said surfacemount passive component.
 20. An alarm system that includes a circuitboard assembly which comprises: an electrically insulating layer; aconductive printed layer formed on the surface of said electricallyinsulating layer and including a plurality of conductive paths; aconductive trace on said electrically insulating layer and means fordissipating a transient; a surface mount resistor fixed in closelyspaced relation to said trace.
 21. The alarm system as described inclaim 20 wherein said surface mount resistor has opposed generallyplanar lower lips.
 22. The alarm system as described in claim 20 whereinsaid surface mount resistor has opposed generally planar lower lips andsaid trace is also generally planar.
 23. The alarm system as describedin claim 20 wherein said surface mount resistor has opposed generallyplanar lower lips, said trace is also generally planar and said lowerlips and said trace are generally parallel.
 24. The alarm system asdescribed in claim 20 wherein said generally planar lower lips of saidsurface mount resistor is closer to said trace than the thickness ofsaid surface mount resistor.
 25. The alarm system as described in claim20 wherein said surfaces of said lips and said trace are parallel. 26.The alarm system as described in claim 20 wherein said generally planarlips of said surface mount resistor are spaced no more than 0.01 inchfrom said trace.
 27. A circuit board assembly which comprises: anelectrically insulating layer; a conductive printed wiring layer formedon the surface of said electrically insulating layer and including aplurality of conductive paths; a conductive trace on said electricallyinsulating layer and means for dissipating a transient; a first surfacemount resistor fixed in closely spaced relation to said trace; a secondsurface mount resistor fixed in closely space relation to said trace.28. The circuit board assembly as described in claim 27 wherein each ofsaid surface mount resistors has opposed generally planar lips.
 29. Thecircuit board assembly as described in claim 27 wherein each of saidsurface mount resistors has a generally planar lips and said trace isalso generally planar.
 30. The circuit board assembly as described inclaim 27 wherein each of said surface mount resistors has a generallyplanar lower lip, said trace is also generally planar and all of saidlower lips and said trace are generally parallel.
 31. The circuit boardassembly as described in claim 27 wherein said generally planar lips ofsaid surface mount resistors are closer to said trace than the thicknessof said surface mount resistors.
 32. The circuit board assembly asdescribed in claim 28 wherein said lips and said trace are parallel. 33.The circuit board assembly as described in claim 32 wherein a singlegeometric plane extends through substantially all of said lips and allof said trace.
 34. A circuit board assembly as described in claim 33wherein the lower surface of said lips and the lower surface of saidtrace are substantially coplanar.
 35. A circuit board assembly asdescribed in claim 33 wherein the upper surface of said lower lips andthe upper surface of said trace are substantially coplanar.
 36. Acircuit board assembly as described in claim 33 wherein the lowersurface of said lower lips and the lower surface of said trace aresubstantially coplanar and in addition the upper surface of said lipsand the upper surface of said trace are substantially coplanar.
 37. Acircuit board assembly as described in claim 33 wherein said surfacemount resistor has a height of t and the spacing between each of saidlips and said trace is less than t.
 38. A circuit board assembly asdescribed in claim 37 wherein the spacing between each of said lips andsaid trace is no more than one half t.
 39. A circuit board assembly asdescribed in claim 27 wherein the distance between said first and secondresistors is greater than the height of each resistor.
 40. A circuitboard assembly as described in claim 28 wherein the distance betweensaid first and second resistors is greater than the height of eachresistor.
 41. A circuit board assembly as described in claim 29 whereinthe distance between said first and second resistors is greater than theheight of each resistor.
 42. A circuit board assembly as described inclaim 30 wherein the distance between said first and second resistors isgreater than the height of each resistor.
 43. A circuit board assemblyas described in claim 31 wherein the distance between said first andsecond resistors is greater than the height of each resistor.
 44. Acircuit board assembly as described in claim 32 wherein the distancebetween said first and second resistors is greater than the height ofeach resistor.
 45. A circuit board assembly as described in claim 33wherein the distance between said first and second resistors is greaterthan the height of each resistor.
 46. A circuit board assembly asdescribed in claim 34 wherein the distance between said first and secondresistors is greater than the height of each resistor.
 47. A circuitboard assembly as described in claim 35 wherein the distance betweensaid first and second resistors is greater than the height of eachresistor.
 48. A circuit board assembly as described in claim 36 whereinthe distance between said first and second resistors is greater than theheight of each resistor.
 49. A circuit board assembly as described inclaim 38 wherein the distance between said first and second resistors isgreater than the height of each resistor.
 50. A circuit board assemblyas described in claim 27 wherein the distance between said first andsecond resistors is at least three times the height of each resistor.51. A circuit board assembly as described in claim 21 wherein thedistance between said first and second resistors is at least three timesthe height of each resistor.
 52. A circuit board assembly as describedin claim 29 wherein the distance between said first and second resistorsis at least three times the height of each resistor.
 53. A circuit boardassembly as described in claim 30 wherein the distance between saidfirst and second resistors is at least three times the height of eachresistor.
 54. A circuit board assembly as described in claim 32 onewherein the distance between said first and second resistors is at leastthree times the height of each resistor.
 55. A circuit board assembly asdescribed in claim 32 wherein the distance between said first and secondresistors is at least three times the height of each resistor.
 56. Acircuit board assembly as described in claim 33 wherein the distancebetween said first and second resistors is at least three times theheight of each resistor.
 57. A circuit board assembly as described inclaim 34 wherein the distance between said first and second resistors isat least three times the height of each resistor.
 58. A circuit boardassembly as described in claim 35 wherein the distance between saidfirst and second resistors is at least three times the height of eachresistor.
 60. A circuit board assembly as described in claim 37 whereinthe distance between said first and second resistors is at least threetimes the height of each resistor.
 60. A circuit board assembly asdescribed in claim 38 wherein the distance between said first and secondresistors is at least three times the height of each resistor.
 61. Acircuit board assembly as described in claim 39 wherein the distancebetween said first and second resistors is at least three times theheight of each resistor.